gen8/cmd_buffer: Don't push CS constants if there aren't any
gen8/cmd_buffer: Don't push CS constants if there aren't any

Issuing MEDIA_CURB_LOAD with a size of zero causes GPU hangs on BDW.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler